1. Field of the Invention
The present invention relates generally to electromagnetic valve driving circuits that drive electromagnetic valves using a high voltage obtained by boosting a supply voltage. More particularly, the invention concerns an electromagnetic valve driving circuit suitable for driving a fuel injector of a direct in-cylinder injection type.
2. Description of the Related Art
Traditionally, in order to improve fuel efficiency and engine power, the automobiles, motorcycles, agricultural tractors, machine tools, and marine engines which are fueled by gasoline, a light oil, or the like, each use an internal-combustion engine controller equipped with an injector that directly injects the fuel into cylinders. Such an injector is called the direct in-cylinder fuel injector or simply the direct injector (DI).
Currently, the scheme for injecting a fuel into an air intake pipe is mainly employed in gasoline engines. Engines equipped with the direct in-cylinder fuel injector that uses the fuel boosted to a high pressure, however, need energy higher than that required for the engines of the above scheme, to open a valve of the injector. In addition, to improve controllability for high-speed rotation, high energy needs to be supplied to the injector. Furthermore, although the technology of multistage injection for saving the fuel and reducing exhaust gas emissions is catching attention in connection with the engines having the direct in-cylinder fuel injector, this technology involves injecting the fuel in several split operations for one piston action, instead of injecting the fuel in one operation in conventional technology, and thus requires supplying high energy to the injector within an even shorter time.
In general, many types of injector driving circuits for controlling the direct in-cylinder fuel injector include a booster circuit that boosts a battery voltage to a higher voltage, and apply the high voltage generated by this booster circuit to reduce an operational response time of the injector. In the multistage injection technology that involves more frequent injector operation than the conventional technology, therefore, the booster circuit increases in load, so it is a critical challenge how to reduce the load of the booster circuit.
A typical current signal waveform of the direct injector is described below. First, during an initial peak-current conduction period of current application, the injector current is boosted to a predetermined peak level by using a boost voltage to open a valve of the injector. This peak current is about 5 to 20 times as great as the injector current developed in the prevailing gasoline engine scheme for injecting a fuel into an air intake pipe. After the conduction period of the peak current, the source of energy supply to the injector changes from the booster circuit to a battery power supply, and thus a valve-opening hold current lower than the peak current level is supplied to hold the open state of the injector valve. When the peak current and the valve-opening hold current are supplied, the injector with the open valve injects the fuel into cylinders.
After the injection, there is a need to cut off the injector current by reducing a level of the injector supply current within a short time to rapidly close the injector valve. However, since the injector current is flowing through the injector and high energy is stored therein, this energy needs to be made to disappear from the injector. In order to implement this within a short time, various schemes are adopted. These schemes include, for example, a scheme that converts the energy into thermal energy by utilizing a Zener diode effect created by a driving element of a circuit which drives the injector current, and a scheme that makes the injector current flow back via a current regeneration diode by providing a boost capacitor having the booster circuit's boost voltage stored therein.
In terms of improving independent characteristics of the injector or combustion characteristics of the fuel in the engine, the injector may preferably hold the peak current for a certain period of time in some cases. The hold of this peak current can be achieved by repeating on/off operations on a switching element connected between the injector and the booster circuit during a short period of time, that is, by intermittently applying the boost voltage to the injector and repeatedly increasing/reducing a slight current. A method likely to be useable to reduce the injector current at this time is by adopting a freewheeling scheme in which the injector current is to be reduced in level by returning the current to a route that passes through a freewheeling diode, or a regenerative scheme in which, as described above, the boost capacitor having the booster circuit's boost voltage stored therein regenerates the injector current during the foregoing valve-closing operation. JP-2008-169762-A, for example, discloses a driving method that uses the freewheeling scheme to hold a peak current of an injector.